Australopithecus afarensis

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Australopithecus afarensis
Temporal range:
Ma
↓
The partial skeleton AL 288-1 ("Lucy")
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Suborder: Haplorhini
Infraorder: Simiiformes
Family: Hominidae
Subfamily: Homininae
Tribe: Hominini
Genus: †Australopithecus
Species:
†A. afarensis
Binomial name
†Australopithecus afarensis
White, and Coppens, 1978[1]
Synonyms
Synonyms

Australopithecus afarensis is an

A. anamensis and is hypothesised to have given rise to Homo
, though the latter is debated.

A. afarensis had a tall face, a delicate brow ridge, and

arboreality), or basal traits inherited from the chimpanzee–human last common ancestor
with no adaptive functionality.

A. afarensis was probably a generalist omnivore of both C3 forest plants and C4 CAM savanna plants—and perhaps creatures which ate such plants—and was able to exploit a variety of different food sources. Similarly, A. afarensis appears to have inhabited a wide range of habitats with no real preference, inhabiting open grasslands or woodlands, shrublands, and lake- or riverside forests. Potential evidence of stone tool use would indicate meat was also a dietary component. Marked sexual dimorphism in primates typically corresponds to a polygynous society and low dimorphism to monogamy, but the group dynamics of early hominins is difficult to predict with accuracy. Early hominins may have fallen prey to the large carnivores of the time, such as big cats and hyenas.

Taxonomy

Research history

Beginning in the 1930s, some of the most ancient

fossil trackways, and preliminarily classified Laetoli remains into Homo spp., attributing Australopithecus-like traits as evidence of them being transitional fossils.[6]

The holotype LH 4

In 1978, Johanson,

holotype specimen because of its preservation quality and because White had already fully described and illustrated it the year before.[1]

Locations of A. afarensis sites

A. afarensis is known only from

KSD-VP-1/1, was recovered from Woranso-Mille.[13]
: 1–4 

For a long time, A. afarensis was the oldest known African

Morotopithecus bishopi.[19]

Classification

A. afarensis is now a widely accepted species, and it is now generally thought that Homo and Paranthropus are

A. anamensis evolved into A. afarensis.[10][20][21]

In 1979, Johanson and White proposed that A. afarensis was the last common ancestor between Homo and

H. rudolfensis) was at first dated to 2.9 million years ago, which cast doubt on the ancestral position of both A. afarensis or A. africanus, but it has been re-dated to about 2 million years ago.[8] Several Australopithecus species have since been postulated to represent the ancestor to Homo, but the 2013 discovery of the earliest Homo specimen, LD 350-1, 2.8 million years old (older than almost all other Australopithecus species) from the Afar Region could potentially affirm A. afarensis' ancestral position.[24] However, A. afarensis is also argued to have been too derived (too specialised), due to resemblance in jaw anatomy to the robust australopithecines, to have been a human ancestor.[25]

Palaeoartist

A. garhi.[27] In 2004, Danish biologist Bjarne Westergaard and geologist Niels Bonde proposed splitting off "Homo hadar" with the 3.2-million-year-old partial skull AL 333–45 as the holotype, because a foot from the First Family was apparently more humanlike than that of Lucy. In 2011, Bonde agreed with Ferguson that Lucy should be split into a new species, though erected a new genus as "Afaranthropus antiquus".[28]

In 1996, a 3.6-million-year-old jaw from

A. deyiremeda, and the recognition of this species would call into question the species designation of fossils currently assigned to A. afarensis.[30] However, the validity of A. bahrelghazali and A. deyiremeda is debated.[31] Wood and Boyle (2016) stated there was "low confidence" that A. afarensis, A. bahrelghazali and A. deyiremeda are distinct species, with Kenyanthropus platyops perhaps being indistinct from the latter two.[32]

African
hominin timeline (in mya)
View references
H. sapiensH. nalediH. rhodesiensisH. ergasterAu. sedibaP. robustusP. boiseiH. rudolfensisH. habilisAu. garhiP. aethiopicusLD 350-1K. platyopsAu. bahrelghazaliAu. deyiremedaAu. africanusAu. afarensisAu. anamensisAr. ramidusAr. kadabba


Anatomy

Skull

Two A. afarensis skulls

A. afarensis had a tall face, a delicate brow ridge, and prognathism (the jaw jutted outwards). One of the biggest skulls, AL 444–2, is about the size of a female gorilla skull.[33] The first relatively complete jawbone was discovered in 2002, AL 822–1. This specimen strongly resembles the deep and robust gorilla jawbone. However, unlike gorillas, the strength of the sagittal and nuchal crests (which support the temporalis muscle used in biting) do not vary between sexes. The crests are similar to those of chimpanzees and female gorillas.[25] Compared to earlier hominins, the incisors of A. afarensis are reduced in breadth, the canines reduced in size and lost the honing mechanism which continually sharpens them, the premolars are molar-shaped, and the molars are taller.[34] The molars of australopiths are generally large and flat with thick enamel, which is ideal for crushing hard and brittle foods.[35]

The brain volume of Lucy was estimated to have been 365–417 cc, specimen AL 822-1 about 374–392 cc, AL 333-45 about 486–492 cc, and AL 444-2 about 519–526 cc. This would make for an average of about 445 cc. The brain volumes of the infant (about 2.5 years of age) specimens DIK-1-1 and AL 333-105 are 273–277 and 310–315 cc, respectively. Using these measurements, the brain growth rate of A. afarensis was closer to the growth rate of modern humans than to the faster rate in chimpanzees. Though brain growth was prolonged, the duration was nonetheless much shorter than modern humans, which is why the adult A. afarensis brain was so much smaller. The A. afarensis brain was likely organised like non-human ape brains, with no evidence for humanlike brain configuration.[36]

Size

Reconstruction of a male (left) and female (right) A. afarensis at the Natural History Museum, Vienna

A. afarensis specimens apparently exhibit a wide range of variation, which is generally explained as marked sexual dimorphism with males much bigger than females. In 1991, American anthropologist Henry McHenry estimated body size by measuring the joint sizes of the leg bones and scaling down a human to meet that size. This yielded 151 cm (4 ft 11 in) for a presumed male (AL 333–3), whereas Lucy was 105 cm (3 ft 5 in).[37] In 1992, he estimated that males typically weighed about 44.6 kg (98 lb) and females 29.3 kg (65 lb) assuming body proportions were more humanlike than apelike. This gives a male to female body mass ratio of 1.52, compared to 1.22 in modern humans, 1.37 in chimpanzees, and about 2 for gorillas and orangutans.[38] However, this commonly cited weight figure used only three presumed-female specimens, of which two were among the smallest specimens recorded for the species. It is also contested if australopiths even exhibited heightened sexual dimorphism at all, which if correct would mean the range of variation is normal body size disparity between different individuals regardless of sex. It has also been argued that the femoral head could be used for more accurate size modeling, and the femoral head size variation was the same for both sexes.[39]

Lucy is one of the most complete Pliocene hominin skeletons, with over 40% preserved, but she was one of the smaller specimens of her species. Nonetheless, she has been the subject of several body mass estimates since her discovery, ranging from 13–42 kg (29–93 lb) for absolute lower and upper bounds. Most studies report ranges within 25–37 kg (55–82 lb).[40]

For the five makers of the Laetoli fossil trackways (S1, S2, G1, G2 and G3), based on the relationship between footprint length and bodily dimensions in modern humans, S1 was estimated to have been considerably large at about 165 cm (5 ft 5 in) tall and 45 kg (99 lb) in weight, S2 145 cm (4 ft 9 in) and 39.5 kg (87 lb), G1 114 cm (3 ft 9 in) and 30 kg (66 lb), G2 142 cm (4 ft 8 in) and 39 kg (86 lb), and G3 132 cm (4 ft 4 in) and 35 kg (77 lb). Based on these, S1 is interpreted to have been a male, and the rest females (G1 and G3 possibly juveniles), with A. afarensis being a highly dimorphic species.[41]

Torso

DIK-1-1 preserves an oval hyoid bone (which supports the tongue) more similar to those of chimpanzees and gorillas than the bar-shaped hyoid of humans and orangutans. This would suggest the presence of laryngeal air sacs characteristic of non-human African apes (and large gibbons).[12] Air sacs may lower the risk of hyperventilating when producing faster extended call sequences by rebreathing exhaled air from the air sacs. The loss of these in humans could have been a result of speech and resulting low risk of hyperventilating from normal vocalisation patterns.[42]

It was previously thought that the australopithecines' spine was more like that of non-human apes than humans, with weak

vertebral centra preserved in Lucy were interpreted as being the T6, T8, T10, T11 and L3, but a 2015 study instead interpreted them as being T6, T7, T9, T10 and L3.[44] DIK-1-1 shows that australopithecines had 12 thoracic vertebrae like modern humans instead of 13 like non-human apes.[45] Like humans, australopiths likely had 5 lumbar vertebrae, and this series was likely long and flexible in contrast to the short and inflexible non-human great ape lumbar series.[13]
: 143–153 

Upper limbs

"Lucy" skeleton

Like other australopiths, the A. afarensis skeleton exhibits a mosaic anatomy with some aspects similar to modern humans and others to non-human great apes. The pelvis and leg bones clearly indicate weight-bearing ability, equating to habitual bipedal, but the upper limbs are reminiscent of orangutans, which would indicate

last common ancestor in the absence of major selective pressures at this stage to adopt a more humanlike arm anatomy.[46]

The shoulder joint is somewhat in a shrugging position, closer to the head, like in non-human apes.

H. floresiensis with a more or less human shoulder configuration and larger A. afarensis specimens retaining the shrugging shoulders show this to not have been the case. The scapular spine (reflecting the strength of the back muscles) is closer to the range of gorillas.[47]

The forearm of A. afarensis is incompletely known, yielding various brachial indexes (

precision grip necessary in using stone tools.[49] However, it is unclear if the hand was capable of producing stone tools.[50]

Lower limbs

The australopith pelvis is

birth canal and thus pelvic inlet width) as an A. afarensis newborn would have had a similar or smaller head size compared to that of a newborn chimpanzee.[51][52] It is debated if the platypelloid pelvis provided poorer leverage for the hamstrings or not.[51]

DIK-1-1
skeleton; notice the diverging left big toe bone

The

heel bone of A. afarensis adults and modern humans have the same adaptations for bipedality, indicating a developed grade of walking. The big toe is not dextrous as is in non-human apes (it is adducted), which would make walking more energy efficient at the expense of arboreal locomotion, no longer able to grasp onto tree branches with the feet.[53] However, the foot of the infantile specimen DIK-1-1 indicates some mobility of the big toe, though not to the degree in non-human primates. This would have reduced walking efficiency, but a partially dextrous foot in the juvenile stage may have been important in climbing activities for food or safety, or made it easier for the infant to cling onto and be carried by an adult.[54]

Palaeobiology

Diet and technology

A. afarensis was likely a

succulents and perhaps creatures which ate those, such as termites. Thus, A. afarensis appears to have been capable of exploiting a variety of food resources in a wide range of habitats. In contrast, the earlier A. anamensis and Ar. ramidus, as well as modern savanna chimpanzees, target the same types of food as forest-dwelling counterparts despite living in an environment where these plants are much less abundant. Few modern primate species consume C4 CAM plants.[55] The dental anatomy of A. afarensis is ideal for consuming hard, brittle foods, but microwearing patterns on the molars suggest that such foods were infrequently consumed, probably as fallback items in leaner times.[56]

In 2009 at Dikika, Ethiopia, a rib fragment belonging to a cow-sized

bovid was found to exhibit cut marks, and the former some crushing, which were initially interpreted as the oldest evidence of butchering with stone tools. If correct, this would make it the oldest evidence of sharp-edged stone tool use at 3.4 million years old, and would be attributable to A. afarensis as it is the only species known within the time and place.[57] However, because the fossils were found in a sandstone unit (and were modified by abrasive sand and gravel particles during the fossilisation process), the attribution to hominin activity is weak.[58]

Society

It is highly difficult to speculate with accuracy the group dynamics of early hominins.[59] A. afarensis is typically reconstructed with high levels of sexual dimorphism, with males much larger than females. Using general trends in modern primates, high sexual dimorphism usually equates to a polygynous society due to intense male–male competition over females, like in the harem society of gorillas. However, it has also been argued that A. afarensis had much lower levels of dimorphism, and so had a multi-male kin-based society like chimpanzees. Low dimorphism could also be interpreted as having had a monogamous society with strong male–male competition. Contrarily, the canine teeth are much smaller in A. afarensis than in non-human primates, which should indicate lower aggression because canine size is generally positively correlated with male–male aggression.[60][61][62]

Birth

Diagram comparing birthing mechanisms of a chimpanzee (left), A. afarensis (middle) and a modern human (right)

The platypelloid pelvis may have caused a different birthing mechanism from modern humans, with the

neonate entering the inlet facing laterally (the head was transversally orientated) until it exited through the pelvic outlet. This would be a non-rotational birth, as opposed to a fully rotational birth in humans. However, it has been suggested that the shoulders of the neonate may have been obstructed, and the neonate could have instead entered the inlet transversely and then rotated so that it exited through the outlet oblique to the main axis of the pelvis, which would be a semi-rotational birth. By this argument, there may not have been much space for the neonate to pass through the birth canal, causing a difficult childbirth for the mother.[63]

Gait

Overview of the S1 trackway (above) and image of the L8 test-pit (below)

The Laetoli fossil trackway, generally attributed to A. afarensis, indicates a rather developed grade of bipedal locomotion, more efficient than the bent-hip–bent-knee (BHBK) gait used by non-human great apes (though earlier interpretations of the gait include a BHBK posture or a shuffling movement). Trail A consists of short, broad prints resembling those of a two-and-a-half-year-old child, though it has been suggested this trail was made by the extinct bear Agriotherium africanus. G1 is a trail consisting of four cycles likely made by a child. G2 and G3 are thought to have been made by two adults.[64] In 2014, two more trackways were discovered made by one individual, named S1, extending for a total of 32 m (105 ft). In 2015, a single footprint from a different individual, S2, was discovered.[41]

The shallowness of the toe prints would indicate a more flexed limb posture when the foot hit the ground and perhaps a less arched foot, meaning A. afarensis was less efficient at bipedal locomotion than humans.[65] Some tracks feature a 100 mm (3.9 in) long drag mark probably left by the heel, which may indicate the foot was lifted at a low angle to the ground. For push-off, it appears weight shifted from the heel to the side of the foot and then the toes. Some footprints of S1 either indicate asymmetrical walking where weight was sometimes placed on the anterolateral part (the side of the front half of the foot) before toe-off, or sometimes the upper body was rotated mid-step. The angle of gait (the angle between the direction the foot is pointing in on touchdown and median line drawn through the entire trackway) ranges from 2–11° for both right and left sides. G1 generally shows wide and asymmetrical angles, whereas the others typically show low angles.[41]

The speed of the track makers has been variously estimated depending on the method used, with G1 reported at 0.47, 0.56, 0.64, 0.7 and 1 m/s (1.69, 2, 2.3, 2.5 and 3.6 km/h; 1.1, 1.3, 1.4, 1.6 and 2.2 mph); G2/3 reported at 0.37, 0.84 and 1 m/s (1.3, 2.9 and 3.6 km/h; 0.8, 1.8 and 2.2 mph);[64][41] and S1 at 0.51 or 0.93 m/s (1.8 or 3.3 km/h; 1.1 or 2.1 mph).[41] For comparison, modern humans typically walk at 1–1.7 m/s (3.6–6.1 km/h; 2.2–3.8 mph).[64]

The average step distance is 568 mm (1.86 ft), and stride distance 1,139 mm (3.74 ft). S1 appears to have had the highest average step and stride length of, respectively, 505–660 mm2 (0.783–1.023 sq in) and 1,044–1,284 mm (3.43–4.21 ft) whereas G1–G3 averaged, respectively, 416, 453 and 433 mm (1.4, 1.5 and 1.4 ft) for step and 829, 880 and 876 mm (2.7, 2.9 and 2.9 ft) for stride.[41]

Pathology

Australopithecines, in general, seem to have had a high incidence rate of vertebral pathologies, possibly because their vertebrae were better adapted to withstand suspension loads in climbing than compressive loads while walking upright.[13]: 95–97  Lucy presents marked thoracic kyphosis (hunchback) and was diagnosed with Scheuermann's disease, probably caused by overstraining her back, which can lead to a hunched posture in modern humans due to irregular curving of the spine. Because her condition presented quite similarly to that seen in modern human patients, this would indicate a basically human range of locomotor function in walking for A. afarensis. The original straining may have occurred while climbing or swinging in the trees, though, even if correct, this does not indicate that her species was maladapted for arboreal behaviour, much like how humans are not maladapted for bipedal posture despite developing arthritis.[66] KSD-VP-1/1 seemingly exhibits compensatory action by the neck and lumbar vertebrae (gooseneck) consistent with thoracic kyphosis and Scheuermann's disease, but thoracic vertebrae are not preserved in this specimen.[13]: 95–97 

In 2010, KSD-VP-1/1 presented evidence of a

fibular fracture during childhood which improperly healed in a nonunion.[13]
: 162–163 

In 2016, palaeoanthropologist John Kappelman argued that the fracturing exhibited by Lucy was consistent with a

mudslide.[68]

The 13 AL 333 individuals are thought to have been deposited at about the same time as one another, bear little evidence of carnivore activity, and were buried on a 7 m (23 ft) stretch of a hill. In 1981, anthropologists James Louis Aronson and Taieb suggested they were killed in a flash flood. British archaeologist Paul Pettitt considered natural causes unlikely and, in 2013, speculated that these individuals were purposefully hidden in tall grass by other hominins (funerary caching).[69] This behaviour has been documented in modern primates, and may be done so that the recently deceased do not attract predators to living grounds.[70]

Palaeoecology

A. afarensis does not appear to have had a preferred environment, and inhabited a wide range of habitats such as open grasslands or woodlands, shrublands, and lake- or riverside forests.

saber-toothed cats: Megantereon, Dinofelis, Homotherium and Machairodus.[72]

Australopithecines and early Homo likely preferred cooler conditions than later Homo, as there are no australopithecine sites that were below 1,000 m (3,300 ft) in elevation at the time of deposition. This would mean that, like chimpanzees, they often inhabited areas with an average diurnal temperature of 25 °C (77 °F), dropping to 10 or 5 °C (50 or 41 °F) at night.[73] At Hadar, the average temperature from 3.4 to 2.95 million years ago was about 20.2 °C (68.4 °F).[74]

See also

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Further reading

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